An image forming apparatus includes an image forming unit that forms images on a transferred medium such as a sheet conveyed on a transferred medium conveying unit such as a belt. In this type of image forming apparatus, positional deviations of images formed on the transferred medium by the image forming unit may become a factor. Particularly, in a color image forming apparatus, if the positions of images in colors of cyan, yellow, and magenta or the like deviate, it appears as so-called color shift and a clear image cannot be formed.
Therefore, it has been proposed that test patterns (also called registration marks) are formed on the transferred medium conveying unit or the transferred medium and passage timings of the test patterns are optically detected by detection sensors to calculate the positions of the images (See, e.g. JP-A-8-278680).
In JP-A-8-278680, on a belt as the transferred medium conveying unit, as illustrated in FIG. 8A, a test pattern TP consisting of a line segment L1 orthogonal to the moving direction of the belt and a line segment L2 slanting with respect to the moving direction is formed. Based on a detection timing difference between the line segments L1 and L2, the positions in a main scanning direction (orthogonal to the moving direction) of the test patterns TP can be detected.
However, if a spot diameter of the detection sensor is in a circle shown by SS in FIG. 8A where the spot diameter is larger than the widths of the line segments L1 and L2, the height of a peak differs between the detection of the line segment L1 and the detection of the line segment L2 in the detection sensor output waveform as shown in FIG. 8B. In this case, a threshold Sh to be used for detection of the line segments L1 and L2 has to be set to a narrow range according to the lower peak height as shown by an arrow in FIG. 8B, so that the detection stability does not deteriorate.
If the spot diameter SS is narrowed by using a slit plate, the height of the peak can be made equal between the detection of the line segment L1 and the detection of the line segment L2, however, the number of parts increases and the manufacturing cost increases. If the spot diameter SS is narrowed, the peak of the output waveform also accordingly is reduced. As a result, there is a possibility that the stability of the detection cannot be greatly improved.